On the Discovery of Fission

Shelly Glashow’s splendid essay is a little like one of those candy-filled Mexican piñatas.1 Each item inside is not only a bit of a surprise, but also a delight. I will focus here on only one thing: Ida Noddack and her fission conjecture. I think that I can supply some useful adumbration.

Ida Noddack, née Tacke, was born in Lackhausen, Germany in 1896. Her father manufactured varnish. She was one of the first women in Germany to take a PhD in chemistry, which she did in 1921, and the first to hold a position as a chemist in industry. She married the chemist Walter Noddack and much of her work was done in collaboration with him; she was largely an unpaid collaborator. They discovered the element rhenium, for which a Nobel Prize was proposed. Some of their other element discoveries were challenged. Nonetheless, they were scientists who had to be taken seriously, which raises the question as to why her 1934 paper “On Element 93,” published in the Zeitschrift für Angewandte Chemie, was not taken seriously.2 This is mentioned in Glashow’s paper and this is what I would now like to discuss in more detail.

In a previous essay, I described the work of Enrico Fermi and his group in Rome and why they failed to discover fission.3 What Fermi thought they had discovered were transuranic elements, such as what was later called plutonium. Noddack claimed that Fermi had not been careful to consider all possibilities. “When heavy nuclei are bombarded by neutrons,” she wrote, “it is conceivable that the nucleus breaks up into several large fragments, which would of course be isotopes of known elements but would not be neighbors of the irradiated element.” What is missing here is any sense of mechanism. Even conservation of energy is not considered.

What has always surprised me about this episode is that none of them—Fermi, Niels Bohr, and Werner Heisenberg—considered the curve of binding energy. Even a high-school student would observe that the curve peaks at iron; the elements in the middle of the periodic table are more tightly bound than uranium, which makes their mass-energy available for fission. This was first noted by Noddack’s contemporary Lise Meitner four years later. It was also accidental that Fermi’s group did not discover fission in spite of themselves; they had installed additional shielding which blocked out the fission fragments.

The German radio chemist Otto Hahn was awarded the 1944 Nobel Prize in chemistry. The war was still on, so the award was not formally delivered until a year later. At the time it was announced in 1945 Hahn was being detained at Her Majesty’s pleasure in a manor house—Farm Hall—near Cambridge. Among the nine other detainees was Heisenberg. Their conversations were recorded by British intelligence. The one about the Nobel Prize is especially delicious. The Swedish Academy had no idea where Hahn was, but the prize was announced in the British newspapers. Hahn and Heisenberg were together and Heisenberg was reading the paper when he found the announcement. One of the things that is entertaining is that the detainees never called each other by their first names. Heisenberg tried to tell Hahn that he had won the Nobel Prize, but at first Hahn seemed slightly annoyed that his thoughts had been interrupted. Soon, however, there were general congratulations and a celebration was arranged in which there was singing by the detainees. In 1944 the existence of the atomic bomb was not known, but in 1945 it was, so this award is especially ironic.

I’d like to focus here on the question of what Hahn actually discovered. In 1938, he was carrying out a series of experiments with his younger colleague Fritz Strassmann. These were along the lines of what Fermi and his group had done in Rome and what Hahn and Strassmann had done with the physicist Lise Meitner from 1934 to 1938. But this time something new appeared. Instead of the transuranic elements they thought they might find, there was barium, an element somewhere in the middle of the periodic table. Hahn was totally mystified. He had no idea how the barium had gotten there. In desperation he wrote to his former Berlin colleague, Meitner. She had Jewish ancestry and had found asylum in Sweden. About the time of the arrival of the letter she had gone on a Christmas vacation in Sweden to be joined by her nephew, the physicist Otto Frisch, who was in exile in Copenhagen at Bohr’s Institute. Frisch had some physics he wanted to discuss with his aunt, but the only thing she would talk about was Hahn’s barium. When Frisch suggested that it might have been a mistake, she told him in no uncertain terms that Hahn did not make mistakes like that. So they went for a walk in the woods, Frisch on skis and his aunt trotting along in the snow. Somewhere in the middle of the walk they understood everything.

Meitner, of course, knew the curve of binding energy, but she had had an assistant, Carl Friedrich von Weizsäcker, who had written down what is known as the semi-empirical formula for nuclear binding energies EB. He published it in 1935. The mass of a nucleus is given by

The Weizsäcker formula for EB, which is needed to find the mass, is called semi-empirical because the various terms are suggested by theory, but the parameters are chosen empirically to best fit the data. They therefore knew the mass of barium. If the nucleus had been split in two, the partner of the barium, by conservation of charge, had to be krypton, whose mass they could also find from the formula—Krypton is an inert gas and had simply floated out of Hahn’s apparatus. They immediately saw that the sum of these masses in energy units was about 200 million electron volts less than uranium. This mass-energy was what the splitting would release. On an atomic scale, this is huge. The binding energy of an electron in hydrogen is only 13.6 electron volts. But on a practical level, it is minuscule. It might have remained an academic curiosity except for the notion of a chain reaction. In addition to fission fragments, neutrons are produced, generally more than two. If these induce further fissions, you have a chain reaction. If this runs away, you have an explosion. There is no mention of this in their short paper.

It is amusing that in this paper the term fission, which is introduced for the first time, is in quotation marks. Frisch got it from a biologist who said it was used in cell splitting. When Frisch returned to Copenhagen, he told Bohr, who immediately saw that it must be correct. Over the years I have made it a practice of asking physicists, like Hans Bethe and Robert Serber, who were around then what their reaction was. Serber said his reaction was: “How could we have been so stupid not to have predicted this?”

Before I return to Noddack’s role, I would like to discuss the matter of Hahn’s Nobel Prize. Did he deserve it? Yes, I would say, but not alone. He should have shared it with Meitner. Although Hahn did not know what he had discovered, nonetheless he had discovered it, and had the conviction to publish it. I see a parallel in this to the 1964 accidental discovery by Arno Penzias and Robert Wilson of what turned out to be the background radiation left over from the Big Bang. What they actually discovered was noise in a radio telescope, which turned out to be from this radiation. They had at the time no idea of this, and indeed Wilson did not even believe in the Big Bang. Nonetheless, they had the courage to publish, so accident or not, they made the discovery and won their Nobel Prize. Now to Noddack.

Let me begin with exactly what she wrote in her paper of 1934:

When heavy nuclei are bombarded by neutrons, it is conceivable that the nucleus breaks up into several large fragments, which would of course be isotopes of known elements but would not be neighbors of the irradiated element.

One could take two polar opposite views of this. On the one hand, one could say this was a significant scientific prediction, which Noddack should be credited with, and, on the other hand, one could say that it was a wild guess for which she offered no scientific explanation.

Unsurprisingly, Noddack took the former position. She had her claim published in Die Naturwissenschaften, the very journal in which Hahn and Strassmann had published theirs.4 Not only did she claim priority for her idea, but she accused Hahn of various experimental inconsistencies. Hahn was beside himself, and wrote an extremely nasty retort which he was persuaded not to publish by the editor. Meitner was informed of all this and wrote to Hahn: “Dass sie eine unangenehme Ursche ist, habe ich immer gewusst.” The English translation is somewhat up for grabs due to her use of the colloquial word Ursche. I would lean toward, “That she was always a total bitch I have always known,” but the reader may wish to supply their own version.

There was certainly something political about Meitner’s views. Germans who lived through the Third Reich fall into three categories: Nazis, non-Nazis, and anti-Nazis. The non-Nazis were people who went along, making whatever compromises they had to. The Noddacks were, at best, non-Nazis. After all, Walter Noddack took the university position of a Jew who had been forced out, like Meitner for that matter. It is difficult to imagine that this did not cross Meitner’s mind. Whether the two women, both of whom were in Berlin, ever met I do not know. Hahn was certainly a non-Nazi, and so was Heisenberg. Strassmann was something else. He and his wife had hidden Jews in their apartment. If they had been caught, the punishment would have been the concentration camp or worse.

By the time of the Nobel award, Hahn had calmed down and even wrote an acknowledgement to Noddack in the printed version of his Nobel address:

From another direction (Ida Noddack) the objection was raised that all the elements of the Periodic System must first be excluded before it was possible to draw the conclusion that an element 93 had been obtained. This objection was not taken seriously as it appeared to be in opposition to all physical views of nuclear physics.

I have always been grateful that fission was not discovered sooner. If Fermi had discovered fission in 1934, the nuclear arms race would have begun then and we might have lost. Our program was much indebted to refugee scientists such as Fermi. Ignoring Noddack did us all a favor. As I mentioned, in 1935 her husband took the position at Freiberg University of a Jewish professor who had been forced to leave. Noddack and his wife never joined the Party, but after the war they were not able to find real academic jobs. Walter died in 1960, but Ida continued her research until 1968, finally dying ten years later.

Jeremy Bernstein

Jeremy Bernstein is professor emeritus of physics at the Stevens Institute of Technology.

The reader may forgive the familiar first name. I have known Shelly since he first came to Harvard as a graduate student. &larrhk;

An English translation of the Hahn-Strassmann paper entitled “Concerning the Existence of Alkaline Earth Metals Resulting from Neutron Irradiation of Uranium” was published in the January 1964 issue of the American Journal of Physics. I would like to thank Michael Pearson and Ruth Sime for their comments. See Ruth Lewin Sime, Lise Meitner: A Life in Physics (Berkeley, CA: University of California Press, 1996). &larrhk;